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Materials Chemistry Accepted Manuscript
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Volume 22 | Number 1 | 7 January 2012 | Pages 1–252
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ISSN 0959-9428
PAPER M. Vallet-Regí et al. Supramolecular mechanisms in the synthesis of mesoporous magnetic nanospheres for hyperthermia
0959-9428(2012)22:1;1-2
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Takayuki Chiba, a Yong-Jin Pu, a * Hisahiro Sasabe, a Junji Kido,* a Yang Yang b 5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/c000000x
We present a solution-based process to fabricate stacked OLEDs consisting of two polymer light-emitting units (LEUs), connected in series by a charge generation layer (CGL). We used Cs2CO3-doped ZnO nanoparticles as an EIL on the LE-polymer to improve the electron injection from the cathode. The surface morphology of a spin-coated metal oxide nanoparticle appears to be rough, with many gaps due to agglutination of nanoparticles. 10 We chose poly(4-vinyl pyridine) (PVPy) as a binder to improve the film morphology of the ZnO:Cs2CO3 mixture and facilitated the formation of a uniform and dense film to prevent the solvent from soaking into the 1st LEU. The efficient solution-based processing of EILs in the 1st CGL containing MoO3 / poly-TPD bilayers was employed for the construction of an MPE device. The device exhibited a sum current efficiency of 10 cd A1 , with 4 cd A-1 contributed by the 1st unit and 6 cd A-1 by the 2nd unit. 15
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1. Introdution In conventional organic light-emitting devices (OLEDs), the luminance increases as the current density increases. Over time, the operational lifetime of these devices decreases because of the degradation of the organic material as the charge passes through the organic layer.1 Thus, it is thought that high luminance and long operating lifetimes cannot be simultaneously achieved in one device. To address this problem, we developed multi PhotonEmission-units (MPE) OLEDs2, 3 comprising several vertically stacked light-emitting units (LEUs) connected in series by a charge-generation layer (CGL) (Figure 1). In the case of two LEUs connected in series, the driving voltage and luminance of the MPE-OLED are the sum of the voltage and luminance, respectively, for the two LEUs, while the current density for two units is the same as that of a single LEU. For this reason, the current efficiency of the MPE-OLED with two LEUs connected in series is twice that of the conventional single-unit OLED. An electric field generates electrons and holes in the CGL that are injected consecutively into the 1st and 2nd LEU and that recombine to generate photons in each LEU. The CGL is composed of electron-accepting materials, such as MoO34, V2O55, and WO36, and electron-donating materials, such as arylamine derivatives. It is important to match the Fermi level of electronaccepting materials and the highest occupied molecular ------------------------------------------------------------------------------a
Department of Organic Device Engineering, Research Center for Organic Electronics, Yamagata University4-3-16 Johnan, Yonezawa, Yamagata 992-8510 (Japan) E-mail:
[email protected];
[email protected] b Department of Materials Science and Engineering, University of California Los Angele, CA 90095, USA. † Electronic supplementary information (ESI) available: See DOI: 10.1039/b000000x/
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orbital (HOMO) level of electron-donating materials.7 Improvement of electron injection from the CGL into the 1st LEU is also important, and an alkali metal-doped electron injection layer (EIL)4, 5, 8 or stack of ultrathin (1 nm) LiF and Al on the electron transporting layer (ETL) of the 1st LEU3 have been employed to improve the electron injection. With regard to the fabrication process, the MPE-OLEDs have only been produced by thermal evaporation processes, although polymer light-emitting devices (PLEDs) that employ the solution-based processes, such as slot die coating and gravure printing, have lower fabrication costs for a large display or lighting area.9, 10 In PLEDs, it is difficult to fabricate well-controlled multilayer structures because of the similar solubility of polymer materials in organic solvents.11-13 In this study, we used a solution-based process to fabricate MPEOLEDs consisting of two polymer light-emitting units connected in series by a CGL. A hybrid process of spin-coating and thermal evaporation was utilized for the fabrication. Each LEU with the configuration of PEDOT:PSS / LE-polymer / EIL was fabricated by spin-coating. A thin (1 nm) layer of Al was deposited as the EIL in the 1st unit, and MoO3 was subsequently deposited as the CGL by thermal evaporation. Low-work-function metals cannot be used as the EIL for solution-based processing of MPE devices because of their high reactivity with organic solvents, which results in severe degradation of the device. To avoid this intrinsic problem, Cs2CO3 has been utilized as an effective EIL material because it is soluble in some alcohol solvents and can be coated from solution. The solution-based processing of Cs2CO3 for use in the EIL exhibits an electron injection ability comparable to those of alkali metals.14, 15 A strong chemical reduction occurrs between Cs2CO3 and the thermally evaporated Al cathode.15 As a result, the thickness of the Cs2CO3 layer must be extremely thin,
Journal of Materials Chemistry Accepted Manuscript
Solution-processed organic light-emitting devices with two polymer light-emitting units connected in series by a charge-generation layer
Journal of Materials Chemistry
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10 cd A-1, with 4 cd A-1 contributed by the 1st unit and 6 cd A-1 by the 2nd unit. The detailed structures of the devices and the chemical structure of the compounds are shown in Table 1 and Fig. 1, respectively. (a)
(b)
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Fig. 1 (a) Chemical structures of the compounds. Device structure of the solution-based process used for fabricating (b) single PLEDs and (c) MPE OLEDs.
Table1. Layer structures of devices. Device or uni ts
Layer structures
A
ITO/PEDOT:PSS (40 nm )/F8BT (100 nm )/Cs 2CO 3 (1 nm )/Al (100 nm )
B
ITO/PEDOT:PSS (40 nm )/F8BT (100 nm )/ZnO:Cs 2CO 3 (10 nm )/Al (100 nm )
C
ITO/PEDOT:PSS (40 nm )/F8BT (100 nm )/ZnO:Cs 2CO 3:PV Py (10 nm )/A l (100 nm )
D
ITO/PEDOT:PSS (40 nm )/EL-Y 1/Al (100 nm )
E
ITO/M oO 3 (10 nm )/EL-Y 2/Al (100 nm )
F
ITO/PEDOT:PSS (40 nm )/EL-Y 1/Al (1 nm )/M oO 3 (10 nm )/EL-Y 2 /Al (100 nm )
EL-Y 1
Poly-T PD (20 nm )/F8BT:Rubre ne (100 nm )/ZnO :Cs 2CO 3:PVPy (10 nm )
EL-Y 2
Poly-T PD (20 nm )/F8BT:Rubre ne (100 nm )/Cs 2CO 3 (1 nm )
Journal of Materials Chemistry Accepted Manuscript
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no thicker than 2 nm, to fully react with the evaporated Al. Conversely, nanocrystalline metal oxides, such as ZnO, have recently been reported as air-stable electron injection materials in PLEDs.16-19 In general, nanocrystalline metal oxide films are fabricated by employing spray pyrolysis deposition onto an ITO substrate using metal oxide precursors. This process requires a high annealing temperature (200 – 450 ºC)16-22 to achieve a highly crystalline state and high mobility of the metal oxides. Consequently, the nanocrystalline metal oxides are often used in inverted PLEDs, in which they are deposited onto a thermally stable ITO substrate and enhance the electron injection from the ITO into the EML. Several studies have used ZnO doped with Cs2CO320 or ZnO with a thin layer of Cs2CO321, 22 for improving the electron injection ability and device performance. However, nonhydrolytic sol-gel processing of the ZnO can be employed to avoid high annealing temperatures and for depositing controlled nanometer-scale particles onto a polymer layer at ambient temperatures.23 Thus, we used Cs2CO3-doped ZnO nanoparticles as an EIL on the LE-polymer to improve the electron injection from the cathode. The surface morphology of a spin-coated metal oxide nanoparticle appears to be rough, with many gaps due to agglutination of nanoparticles. Consequently, the thin layer of metal oxide nanoparticles cannot protect the 1st LEU organic layer from the spin-coating solvent of the 2nd LEU organic layer. We chose poly(4-vinyl pyridine) (PVPy) as a binder to improve the film morphology of the ZnO:Cs2CO3 mixture and facilitated the formation of a uniform and dense film to prevent the solvent from soaking into the 1st LEU. The thermally evaporated MoO3 layer is insoluble in organic solvents, such as toluene, p-xylene and dichlorobenzene. Thus, an electron-donating layer can be spin-coated on top of the MoO3 layer. Poly(4-butylphenyldiphenyl-amine) (poly-TPD) was used as an electron-donating and hole-transporting layer, and was spin-coated onto the MoO3 layer using a dichlorobenzene solution. This combination of MoO3 and poly-TPD as a prospective CGL may be successful because bilayers of MoO3 and arylamine derivatives, such as NPD or TPD, can work as an efficient CGL in MPE OLEDs.4 Poly-TPD is insoluble in toluene and p-xylene; therefore, an LEpolymer such as poly(9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT) can be spin-coated onto the poly-TPD layer using a pxylene solution without dissolving the bottom layer. The efficient solution-based processing of EILs in the 1st CGL containing MoO3 / poly-TPD bilayers was employed for the construction of an MPE device. The device exhibited a sum current efficiency of
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2. Result and discussion
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The EILs that consisted of Cs2CO3, ZnO:Cs2CO3, and ZnO:Cs2CO3:PVPy were formed without using a high annealing temperature. The ZnO nanoparticles, synthesized from a zinc acetate precursor,24 were well dispersed into 2-ethoxyethanol at a concentration of 1.0 %w·v-1. The average diameter of the monodispersed ZnO nanoparticles, as estimated by transmission electron microscopy (TEM), was approximately 8 nm. The Cs2CO3-doped ZnO was obtained by mixing a 1%w·v-1 ZnO dispersion and a 0.2%w·v-1 Cs2CO3 solution at a 1:1 volume ratio. The ZnO:Cs2CO3:PVPy was also obtained by blending the individual 1%w·v-1 ZnO dispersion, 0.2%w·v-1 Cs2CO3 solution, and 0.2%w·v-1 PVPy solution at a 1:1:1 volume ratio. PVPy was dissolved in an alcohol solvent, such as 2-ethoxyethanol, ethanol and isopropanol. These ZnO:Cs2CO3 and ZnO:Cs2CO3:PVPy solutions are stable for more than 6 months without precipitating ZnO. Fig. 2a shows the current density–voltage characteristics of the MPE devices. Device B with ZnO:Cs2CO3 exhibited a driving voltage of 3.2 V at 100 cd m-2 and 4.2 V at 1000 cd m-2. These values are lower than the driving voltages obtained with device A with a Cs2CO3 EIL. Device C, containing ZnO:Cs2CO3:PVPy, exhibited driving voltages of 3.4 V at 100 cd m-2 and 4.5 V at 1000 cd m-2, which are similar to the performance of device B. Although PVPy is an insulating polymer, device C showed good electron injection characteristics compared with the device without PVPy. The current efficiency-current density-power efficiency plots are shown in Fig2b. At the practical luminance of 1000 cd m-2, device B exhibited a current efficiency of 10.0 cd A1 and a power efficiency of 7.6 lm W-1. The efficiencies of device C were 10.6 cd A-1 for the current efficiency and 7.4 lm W-1 for the power efficiency. These results demonstrate that the PVPy binder is electrochemically inert and does not deteriorate the efficiency of the devices. The surface morphology of the EILs that covered the underlying F8BT film in the presence and absence of a PVPy binder was investigated by atomic force microscopy (AFM) in tapping mode (Fig. 3). F8BT films exhibited a smooth surface with an average surface roughness (Ra) of 0.26 nm. The ZnO:Cs2CO3 film showed a higher Ra of 2.52 nm, and its domain size was approximately 50 nm. In contrast, the ZnO:Cs2CO3 film with the PVPy binder showed a lower Ra of 0.367 nm and a more uniform surface than the film without the PVPy binder. These results demonstrated that the addition of the polymer binder dramatically improved the film morphology without compromising the performance of the device. We used a solution-based process to fabricate MPE OLEDs with ZnO:Cs2CO3:PVPy as the EIL. A rubrene was used as a dopant, having strong hole trapping property in order to confine an emission zone in the interface of HTL and EML and improve charge balance. Yellow-emitting MPE device F, single device D corresponding to the 1st unit, and device E corresponding to the 2nd unit were fabricated. The intensity of the
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electroluminescence (EL) spectra at a current density of 50 mA cm-2 in MPE device F was obtained as the sum of the intensity of each single unit, as shown in Fig 4a. The EL spectrum of the MPE device F showed the peak at 558 nm derived from an emission of rubrene, and exhibited no emission from poly-TPD. This result indicates that the hole and electron generation is completely confined within the EML, and recombination of charge carriers takes place only in the EML. At high luminance values that indicated that the thermally evaporated Al / MoO3 film (particularly the Al) was affected by oxidation, the spincoating solvent of the 2nd LEU organic layer, and the annealing processes (Fig. 4b). At high luminance values of 1000 cd m-2, device D and device E exhibited efficiencies of 6 cd A-1 and 4 cd A-1, respectively, as shown in Fig. 4c. (a)
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Fig. 2 (a) Current density-voltage-luminance plots. (b) Current efficiency-current density-power efficiency plots. Device A (circle), device B (triangle) and device C (square). (a)
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(d)
Fig. 3 AFM images of the electron injection layer on top of the emissive layer. (a) ZnO:Cs2CO3. (b) ZnO:Cs2CO3:PVPy. Downloaded by University of California - Los Angeles on 15 September 2012 Published on 07 September 2012 on http://pubs.rsc.org | doi:10.1039/C2JM35344J
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Fig. 4 (a) EL spectra of device D, device E and device F at a current density of 50 mA/cm2. (b) Current density-voltage plots. (c) Luminance-voltage plots. (d) Current efficiency-voltage plots. Device D (circle), device E (triangle) and device F (square). The efficiency of device E was lower than that of the 1st unit due to the decrease in the charge balance and the increase in the driving voltage for MoO3 as an HIL. Nevertheless, the current efficiency of the MPE device increased to 10 cd A-1, which is the sum of the efficiency of the two single devices. The LUMO level of poly-TPD was 2.3 eV, which was just shallow enough to block electrons from the 2nd LEU to the 1st LEU.25 The conduction band (CB) of ZnO was 7.4 eV25, which was deep enough to block holes from the 1st LEU to the 2nd LEU. Thus, the emissions are attributed to the recombination of charges that were generated in the CGL without current leakage. These results demonstrate that MoO3 / poly-TPD can function as an effective CGL. This solution-based processing of MPE OLEDs achieves almost twice the efficiency of MPE OLEDs fabricated by conventional methods. Furthermore, these findings demonstrate that the CGL worked efficiently. Poly-TPD was used as the HTL in the 1st LEU and as an electron-donating layer in the CGL. We were not able to apply the ZnO:Cs2CO3 to the EIL of the 1st LEU because the spincoating solvent containing poly-TPD and dichlorobenzene soaked into the 1st unit due to the high roughness of the ZnO:Cs2CO3 layer. Ten nanometers of thermally evaporated MoO3 film may have been insufficient to fully cover the ZnO:Cs2CO3 layer because of its considerable roughness. In contrast, ZnO:Cs2CO3:PVPy films are uniform, and 10 nm of MoO3 can be uniformly deposited over the entire EIL area. This result indicates that the morphology of the EIL in the 1st LEU is important for effectively spin-coating the 2nd LEU and achieving suitable MPE OLEDs from solution-based processing.
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We investigated the solution-based processing of an MPE device composed of ZnO:Cs2CO3:PVPy as the EIL in the 1st unit and a MoO3 / poly-TPD multilayer as the CGL. Although PVPy is an insulating polymer, the device with ZnO:Cs2CO3:PVPy as the EIL exhibited a low driving voltage and a high efficiency. Moreover, the ZnO:Cs2CO3:PVPy film appeared to be less rough and to have a more uniform surface than the layer that did not contain a PVPy binder. These findings suggest that the addition of a polymer binder can dramatically improve the film morphology without compromising the performance of the
Journal of Materials Chemistry Accepted Manuscript
(b)
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Journal of Materials Chemistry
device. In addition, we fabricated the MPE device by using a solution-evaporation hybrid process with ZnO:Cs2CO3:PVPy as an EIL. The MPE device showed almost twice the current efficiency of each LE unit. These results demonstrate the advantage of the MPE device and that the fabrication of the device using a solution-based process was successful.
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4. Experimental
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All devices were grown on glass substrates precoated with indium tin oxide (ITO) with a sheet resistance of 20 Ω per square. The substrates were cleaned with ultra-purified water and organic solvents and then dry-cleaned for 30 min by exposure to a UVozone atmosphere. PEDOT:PSS (from Starck) film (thickness ~ 40 nm) was spin-coated on the ITO substrates and annealed at 120 °C in air for 10 min. Poly-TPD (from American Dye Source) layers were spin-coated from a dichlorobenzene solution (6 mg ml-1) and annealed at 130 °C for 10 min. F8BT and 1 wt% rubrene-doped F8BT were spin-coated using a p-xylene solution (12 mg ml-1) and annealed at 130 °C for 10 min. Cs2CO3 (from Sigma Aldrich) was spin-coated on the emissive polymer from 2ethoxyethanol (2 mg ml-1). ZnO nanoparticles were synthesized as described below. First, zinc acetate, water and methanol were added to a flask and heated at 60 °C. Next, potassium hydroxide was dissolved into methanol and added to the flask. The mixture was stirred for 2 h and 15 min. The ZnO solution was prepared by dispersing it in 2-ethoxyethanol. The ZnO:Cs2CO3 and ZnO:Cs2CO3:PVPy solutions were obtained by blending the ZnO solution with the Cs2CO3 and PVPy solutions in a 1:1:1 volume ratio. The MoO3 and Al were thermally evaporated under vacuum (~10-4 Pa). The active area was 12 mm2. The thicknesses of spincoated layer were determined by a Veeco Dektak 8 surface profilometer. The HOMO energy levels were determined by atmospheric ultraviolet photoelectron spectroscopy (Rikken Keiki AC-3). The LUMO energy levels were calculated from the HOMO energy levels and the lowest energy absorption edges of the UV-vis absorption spectra. The current–voltage and light– voltage curves were recorded with a Keithley 2400 sourcemeasure unit and a calibrated silicon photodiode. The luminance and EL spectra were further measured by a Photo Research PR650 spectrophotometer. All devices were tested under a nitrogen environment.
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Acknowledgements This study was supported by the "Printed OLEDs Project" of the Japan Science Technology Agency (JST).
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